1. Field of the Invention
The invention relates to electric clothes dryers and more particularly to the airflow paths within the dryer.
2. Description of the Related Art
Laundry areas in homes are moving out of the basement. The trend in many newer homes is to integrate complete laundry centers into the living space, typically at a considerable expense. Combining this trend with a greater awareness of energy conservation, especially water conservation, has led to a significant increase in demand for high-end front-loading washing machines with many consumer-focused performance attributes. Meanwhile, in general, the clothes dryer has changed very little.
Most standard electric dryers available today in North America operate with a resistance-type heating element at a fixed wattage and a fixed airflow rate (varying only in response to load size and venting configurations that create a system pressure drop). The element typically operates in an on/off mode as determined by the cycle chosen and the temperature of the exhaust flow. Many dryers integrate a type of moisture sensor, the most common being conductivity strips, to assist in identifying when to terminate a cycle. These strips serve to measure gross detection of whether the load is wet or not and typically do not quantify how wet or how dry.
Heat pump dryers are more common in Europe and do a very good job of delivering significant energy savings. A schematic diagram, illustrating the typical components of a heat pump dryer 10, is shown in FIG. 1. The heat pump dryer 10 comprises a compressor 12 with a refrigerant circulation loop 14 connected to a condenser 16 and an evaporator 18. The refrigerant circulating in the refrigerant circulation loop 14 is typically R-22. Air flow is generated by a fan 20 and follows an airflow path 22 through a drum 24 and a lint filter 26. The air is treated in the evaporator 18 and the condenser 16, and then typically recirculated to the fan 20. The evaporator 18 removes a significant portion of the moisture in the air before it flows through the condenser 16 and back to the fan 20.
FIG. 2 schematically shows airflow through the drum 24. Process air from the fan 20 is hot and dry as it enters the drum 24 at an inlet 28, normally at the rear of the drum. The process air interacts with the clothes in the load where it picks up moisture from the load. The moist air, still warm, then exits the drum 24 through an outlet 30, usually at the front of the drum, where it proceeds through the lint filter 26.
Heat pump dryers tend to have very long dry times, which make them unacceptable in the North American market. In some cases, the dry time for a large load is almost twice as long in a heat pump dryer as it is in a standard U.S. electric dryer. There is a need for more energy efficient clothes dryers in North America. One solution is to provide an energy efficient heat pump dryer that delivers reasonable dry times.
Reducing dry times in heat pump dryers might be accomplished, for example, by increasing inlet air temperature and/or increasing airflow through the drum. But these solutions lead to other problems. Air temperatures from a heat pump are essentially limited by the choice of refrigerant. Standard R-22 based heat pump systems do not lend themselves to delivering high temperatures. Under the conditions expected in a clothes dryer, maximum R-22 condenser temperatures are around 150° F. To achieve faster dry times in a heat pump dryer based on higher air temperature, the condenser temperature needs to be higher.
Higher airflow through the drum is easily achievable, but clothes plastering to the outlet grill in the dryer's drum is a significant concern. Standard dryers operate with approximately 100 cfm of airflow. As airflow exceeds 200 cfm, increased pressure drop across the drum tends to cause clothing in the drum to be drawn onto the outlet grill and held in place, which creates a significant problem in maintaining adequate air flow. Added to this complication is the likelihood of lint migration beyond the lint screen into the region of the heat exchangers due to increased airflow. This creates problems for system geometry and the design of the heat exchangers and heat transfer apparatus.
These problems are not limited to heat pump dryers. In conventional electric dryers, increasing air temperature comes at the cost of greater energy expenditure. Also, the problem of clothes plastering to the outlet grill with increased airflow remains.